The present invention relates to an orientation guide. In particular, embodiments of the present invention relate to an orientation guide that can be coupled to a surgical instrument in order to assist in correctly orientating the instrument relative to a patient.
During surgical procedures it is commonly necessary to accurately locate prostheses and instruments. For instance, during implantation of an acetabular cup into a pelvis, it is important to ensure that the cup is accurately located within a reamed cavity. Typically, such a procedure requires a number of separate instruments to be used. It is important to ensure that the alignment determined for the cup during the initial surgical steps is maintained during later surgical steps. In particular, it is important to insure that each surgical instrument is orientated correctly relative to the first used surgical instrument. For instance, the required position of an acetabular cup may be initially determined. The cavity must then be reamed. It is important that the desired varus-valgus angle of the cup is maintained during the reaming by ensuring that the reamer is orientated correctly relative to the instrument used to determine the initial position of the cup. The varus-valgus angle for a patient who is lying on a horizontal operating table such that their long axis is aligned horizontally corresponds to the inclination angle relative to a horizontal plane of a longitudinal axis of an instrument which is aligned with the axis of the cup. Preferably, any rolling movement of the instrument about its longitudinal axis should also be detected and corrected.
The varus-valgus angle must then be maintained during the implantation of the cup. Typically, the cup is coupled to an elongate instrument having an impaction surface at the opposite end. The impaction surface is subjected to an impaction force to drive the cup into the cavity. It is possible that the varus-valgus angle could drift during the impaction.
WO-2005/009303 discloses an orientation device for a surgical instrument. The orientation device may be used for orientating a surgical instrument during the implantation of a surgical cup. A frame is provided which couples to a surgical instrument. The frame includes a spirit level, in particular a bull's-eye spirit level, which is arranged to indicate when the spirit level is lying in a horizontal plane by a bubble being positioned within an inner circle. The frame is flexible such then when the instrument is in a required orientation the frame can be deformed until the spirit level lies in a horizontal plane. Further movement of the instrument causing the orientation of the instrument to change (either through rolling motion about the longitudinal axis of the instrument or through a change in the inclination of the instrument) is detectable by observing the position of the bubble within the spirit level.
However, spirit levels lack sensitivity to angular movement beyond a limited preset angular range. In general, spirit levels, including bull's-eye spirit levels are only able to indicate whether they are positioned horizontally and not indicate the magnitude of any difference from the horizontal.